1. Field of the Invention
The present application relates to optical metrology, and more particularly to azimuthal scanning of a structure formed on a semiconductor wafer.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine a feature of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a grating array in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the grating array. By determining the profile of the grating array, the quality of the fabrication process utilized to form the grating array, and by extension the semiconductor chip proximate the grating array, can be evaluated.
However, when performing optical metrology on a structure, measurement errors may occur if the structure and the incident beam are not properly aligned azimuthally. In particular, cross polarization components of the diffracted beam may complicate the signal measurements, and cause mis-fitting between the measured signals and the analysis model used in optical metrology.
Additionally, optical metrology of three dimensional (3-D) structures, e.g., grating arrays with a dimensionality in two directions, such as contact hole arrays, are increasingly being used in the semiconductor industry. Due to the additional dimension compared to two dimensional (2-D) structures, such as lines/spaces, performing optical metrology of 3-D structures is more complex. For example, in optical metrology of 2-D structure, the critical dimension (CD) in one lateral direction is primarily of interest. In contrast, in optical metrology of 3-D structures, besides the CD, the shape (from a bird's view), the CD ratio, and the orientation of the structures are of interest.